It is well-known in the art that nickel-cadmium batteries have a -.DELTA.V characteristic during charging, where the battery reaches a peak voltage and then continues to decrease in voltage after it is fully charged.
Prior art battery chargers that have both fast and slow or trickle charge circuits use various methods for stopping the fast charge and initiating the slow charge. In some cases, a timer is used to shut off the fast charge after a predetermined period of time. However, in this case, the exact amount of time necessary to charge the battery depends upon its discharged state. Other devices utilize a voltage comparator and the battery voltage is compared with some preset value of voltage and at that point the fast charge is terminated and the slow charge is initiated. Other devices turn off the charging circuit when the battery reaches a predetermined charge voltage for a specific period of time.
The present invention allows a nickel-cadmium battery to be charged to its fully-charged voltage and at that point the fast charging circuit is disconnected and the slow charging circuit is connected to the battery. This battery charger utilizes the concept of the -.DELTA.V characteristic of nickel-cadmium batteries. In this case, the nickel-cadmium battery reaches a peak voltage and begins to decrease in voltage after it is fully charged. The present invention detects the sudden decrease in voltage and disconnects the fast charging circuit and couples the slow or trickle charging circuit to the battery at that point. In addition, the circuit includes expiration of a preset time period for disconnecting the fast charging voltage circuit in the event the -.DELTA.V circuit failed to operate, and also includes a thermostat which opens the fast charge circuit when it reaches 40.degree. to 60.degree. C. and connects the slow charge circuit to the battery.
Thus, with the present invention, a 13 microsecond charging pulse at 22 KHz provides five amps at the fast charging rate and 50-300 milliamps at the slow charging rate by reducing the frequency of the 13 microsecond pulse to about 1.5 KHz. The charging control is accomplished by the characteristic -.DELTA.V of the ni-cad pack at the onset of overcharge, the expiration of the 18-minute timer, or the temperature of the battery as determined by the thermostat at 40.degree.-60.degree. C. The occurrence of any one of those events causes the nominal five amp fast charge rate to drop to the required 50-300 milliamp continuous charge rating of the four-, six- and ten-cell ni-cad battery packs.
Thus, it is an object of the present invention to provide a battery charger circuit that provides a fast charging rate of five amps utilizing 13 microsecond pulses at 22 KHz and a slow charging rate of 50-300 milliamp continuous charge rating of the ni-cad battery packs.
It is also an object of the present invention to utilize the characteristic -.DELTA.V of the ni-cad pack at the onset of overcharge to disconnect the fast charging circuit and connect the slow charging circuit to the battery.
It is still another object of the present invention to use a timer to disconnect the fast charging circuit after a predetermined period of time and couple the slow charging circuit to the battery pack.
It is still another object of the present invention to utilize a thermostat to disconnect the fast charging circuit from the battery when the battery temperature reaches the range of 40.degree.-60.degree. C. and couple the slow charging circuit to the battery.